1. Field of the Invention
This invention relates to polycrystalline silicon which is usable as silicon substrates for solar cells, and a method for producing the polycrystalline silicon.
2. Related Art
From 1995 onward, the global market of solar cells has been largely developed per about 30% every year. In 2001, the worldwide amount of the solar cell production was 390 MWp, which is almost close to 400 MWp. The large development of the global market of solar cells is mainly originated from the development for residential application. As of now, various solar cells are used for various applications, and in 2001, according to Mr. P. Haycock, crystalline silicon solar cells are much used which are made from Cz single crystalline silicon crystal or casted polycrystalline silicon. Therefore, in the global market of solar cells, the ratio of the Cz single crystalline silicon solar cells or the casted polycrystalline silicon solar cells for all of the silicon solar cells has increased. In addition, in view of the single crystalline silicon solar cells and the polycrystalline silicon solar cells, the ratio of the crystalline silicon solar cells for all of the solar cells increased up to 83%. In view of electrical power use, the crystalline silicon solar cells have been dominant.
About 50% of the crystalline silicon solar cells is made from polycrystalline silicon by means of casting method. Concretely, the casted polycrystalline silicon is utilized as p-type polycrystalline silicon by doping B therein to form the substrates of solar cells.
In the B-doped p-type polycrystalline silicon, however, the optical and electrical properties such as of minority-carrier lifetimes may be deteriorated due to the irradiation of sunlight. Therefore, if such a deteriorated polycrystalline silicon is employed as a substrate of a solar cell, the short-circuit current density and the open-circuit voltage of the solar cell may be also deteriorated, so that the various device performances of the solar cell such as conversion efficiency may be degraded.